1. Field of the Invention
The present invention relates to a method and an apparatus for absorbing (meth)acrylic acid, and more particularly to a method and an apparatus for absorbing (meth)acrylic acid, in both of those which the concentration of (meth)acrylic acid in an absorption column bottom solution discharged from the absorption column is controlled by means of changing the water amount contained in a gas which is exhausted from the absorption column.
2. Description of the Prior Art
Easily-polymerizable compounds, such as acrylic acid and methacrylic acid, are used as a raw material for producing industrial products and are produced in a large scale in a big plant. In general,these compounds are manufactured via various absorbing and purifying processes in order to obtain them in a highly-purified state.
For example, in the process of manufacturing acrylic acid, when propylene, propane, acrolein and the like are subjected to a catalytic gas-phase oxidation with gas containing oxygen in the molecular state in the presence of an oxidizing catalyst, substances with low boiling points such as acetic acid, lower aldehydes or water and substances with high boiling point such as furfural or maleic anhydride are generated as the by-products in addition to acrylic acid as the target compound. Consequently, the mixed gas obtained is led into an absorption column for absorbing acrylic acid (hereinafter referred to as xe2x80x9cabsorption columnxe2x80x9d), where the mixed gas is contacted with absorbent to thereby absorb and collect acrylic acid into the absorbent. Thus, a solution containing acrylic acid and the other by-products is obtained. Then, acrylic acid is separated and purified from the solution by employing distillation, stripping, extraction, crystallization techniques or the like so that acrylic acid is obtained as a commercial product.
Such a separation and purifying process is disclosed in the U.S. Pat. No. 5,785,821, where a mixed gas obtained by oxidizing propylene or the like by way of catalytic gas-phase oxidation is led into the acrylic acid absorption column, contacted with the absorbent to thereby obtain the acrylic acid aqueous solution of the mixed gas (also referred to as a xe2x80x9cbottom solutionxe2x80x9d), and the obtained acrylic acid aqueous solution is led into an azeotropic distillation column wherein acrylic acid is obtained.
Since water is inexpensive, has high capability for absorbing (meth)acrylic acid and acrylic acid solution having relatively high concentration can be obtained, an absorbent containing water as the main component is generally used in the foresaid absorbing process for absorbing (meth) acrylic acid from a gas containing (meth)acrylic acid. Further, in the foresaid absorbing process, since almost no particular attention has been paid with regard to the pressure and temperature in the top portion of the absorption column in the past, there is substantially no change in the water amount contained in the gas exhausted from the top portion of the absorption column.
In general, in the process of the catalytic gas-phase oxidation, the air is used as the gas containing oxygen in the molecular state because it is inexpensive. However, when the air is used, due to changes in the atmospheric conditions or the like, variation in the water amount contained in the air to be delivered into the reactor is inevitable. Accordingly, the water content in the gas containing acrylic acid to be introduced into the absorption column fluctuates as well. As a result, the concentration of (meth)acrylic acid in the bottom solution, which is discharged from the bottom of the absorption column, varies. This variation in the concentration of (meth)acrylic acid affects the stabilized operation for separating and purifying (meth)acrylic acid in the subsequent steps onward or cause variation in the amount of waste water.
As a countermeasure for the above problem, it has been proposed to adjust the water concentration of the air to be used in the catalytic gas-phase oxidation process at a constant concentration level. However, the implementation of such countermeasure is not satisfactory and feasible since expenditures for installing such moisture adjusting facility and utilities are too much burden.
As the alternative countermeasure in order to stabilize the (meth)acrylic acid concentration of bottom solution, there is an idea to control the supply amount of the absorbent introduced into the absorption column corresponding to the water amount fed into a reactor that performs the catalytic gas-phase oxidation process. However, when the waste water is recovered and recycled to be used as the absorbent, if it is intended to maintain the volumetric ratio of the waste water with respect to the supply amount of the absorbent (hereinafter referred to as xe2x80x9crecovery ratio of the absorbentxe2x80x9d) to be a constant ratio, it is inevitable to change the amount of the waste water to be recovered and recycled in response to the change in the supply amount of the absorbent. As a result, it becomes inevitable to change the amount of the waste water to be thrown out.
Even though an adjustment of said changes in the waste water amount by way of changing the recovery ratio is attempted, on the other hand, it is required to generally set the recovery ratio at a low ratio since the upper limit of the recovery ratio is fixed to 100%, whereby the amount of the waste water to be finally thrown out increases.
Therefore, as described above, the countermeasures having been implemented in the past for securing the stabilized operation of the process including the waste water processing facility against the effect of the changes in the atmospheric conditions and the like was not satisfactory.
The inventor of the present invention had examined in detail about the supply amount of the absorbent, the amount of the waste water, the concentration of (meth)acrylic acid in the bottom solution and the changes in the absorbing coefficient in order to find a countermeasure for securing the stabilized operation including the waste water processing facility in the subsequent steps against the changes in the atmospheric conditions and the like. As a result, it was found out that all of the problems can be settled by adjusting the water amount in the gas exhausted from the top portion of the absorption column.
Specifically, although the amounts of (meth)acrylic acid and the other by-products fed out of the reactor are substantially constant since the conditions for the catalytic gas-phase oxidation reaction is constant, only the water content changes according to the changes in the atmospheric conditions and the like. Consequently, when the supply amount of the absorbent is fixed at a constant one, maintaining of the concentration of (meth)acrylic acid in the bottom solution causes to maintain the flow rate of the bottom solution, whereby resulting in stabilization in the amount of the waste water. Moreover, surprisingly, the absorbing method according to the present invention leads to less variation in the absorbing coefficient comparing to the method of changing the supply amount of the absorbent.
Accordingly, with the method according to the present invention, it is possible to secure stabilized operation in the subsequent steps including the waste water processing facility while suppressing the variation in the loss of (meth)acrylic acid from the absorption column and from the waste water to the minimum level.
The absorbing method for (meth)acrylic acid according to the present invention that can exert the above-described operations and effects is that, in a manufacturing process for (meth)acrylic acid including steps adapted to manufacture a gas containing (meth) acrylic acid by employing a catalytic gas-phase oxidation reaction and to subsequently introduce the gas into the absorption column for absorbing a target product in a form of the solution by using an absorbent containing water as the main component, characterized in that the concentration of (meth)acrylic acid in a bottom solution discharged from an absorption column is controlled by changing the water amount contained in the gas which is exhausted from the top portion of the absorption column.
In the above-described process, it is preferable to change the water amount contained in the gas exhausted from the top portion of the absorption column by the temperature and pressure in the top portion of the absorption column and to recycle a part or the whole of the waste water drained during the (meth)acrylic acid manufacturing process for use as the absorbent.
In addition, such operation to change the water amount contained in the gas exhausted from the top portion of the absorption column is preferably carried out corresponding to the water amount contained in the gas which is introduced into the reactor for the catalytic gas-phase oxidation in said catalytic gas-phase oxidation reaction or the water content contained in the air.
Further, the apparatus for absorbing (meth)acrylic acid according to the present invention is characterized in being an absorption column for carrying out the above-described process and including means for controlling the temperature in the top portion of the absorption column and/or means for controlling the pressure in the top portion of the same.